Chromosomes, found in the centre of our cells, are the structures that house our genetic information (genes). The ends of the chromosomes are called telomeres. Every time a cell divides in two to make a copy of itself, the telomeres become slightly shorter. Over time, as people age, researchers have found that their cells have shortened telomeres compared to those of younger people.
Shrinking telomeres in the lab
In lab experiments, cells with shortened telomeres do not function as well as cells with longer telomeres.
Researchers have found shortened telomeres in the following groups of people when they examined cells of the immune system:
- HIV-positive people who were not taking potent combination anti-HIV therapy (commonly called ART or HAART)
- people with other chronic viral infections, such as those caused by members of the herpes virus family—CMV (cytomegalovirus) and EBV (Epstein-Barr virus)
- people with certain inflammatory conditions, such as rheumatoid arthritis and type 1 and 2 diabetes
- some people who developed cancer
Why are shortened telomeres a problem?
Every time you have an infection, inflammation occurs and cells of the immune system are activated and mobilized. T-cells divide several times, forming large numbers of cells to contain and fight the infection. Every time T-cells in the blood divide to form new cells, their telomeres become shorter and these cells become less effective at doing their job of protecting the body from infections and cancers.
With short-term viral infections, telomeres do not significantly shrink in T-cells because such cells can produce an enzyme (called telomerase) that helps maintain the length of telomeres. However, over time, the effectiveness of telomerase wanes as people age and so their telomeres shorten.
Shortened telomeres, HIV and nukes
Telomerase contains a small amount of an enzyme called reverse transcriptase (RT). This enzyme is also used by HIV-infected cells. Nukes (nucleoside reverse transcriptase inhibitors) and non-nukes interfere with RT. Therefore, some researchers think that nukes (and possibly non-nukes) interfere with telomerase and its ability to maintain the length of telomeres in cells of the immune system, particularly CD8+ cells. Such cells are the body’s main means of fighting virus-infected cells and tumours.
The Australian team obtained blood samples from HIV-positive participants who were taking ART containing the following nukes:
- 3TC (lamivudine), also found in Kivexa and Trizivir
- tenofovir and FTC (Truvada), also found in Atripla, Complera and Stribild
- abacavir (Ziagen), also found in Kivexa and Trizivir
- AZT (zidovudine, Retrovir), also found in Combivir and Trizivir
The researchers tested varying concentrations of these nukes in cultures of immune cells and found that all drugs inhibited the enzyme telomerase. However, tenofovir was the only nuke to do so at concentrations that can be achieved with oral use. Furthermore, cells grown in the lab in the presence of tenofovir had shortened telomeres.
Results—PEP and telomeres
In cases of possible exposure to HIV, doctors can prescribe a combination of anti-HIV drugs that is taken for 28 consecutive days. This is called post-exposure prophylaxis (PEP). Generally, if PEP is taken within 72 hours of exposure to HIV, it can help to contain the virus and help keep the person from becoming infected. PEP combinations vary from one region to another but generally consist of between two and three anti-HIV agents.
The Australian team collected blood samples before, during and after PEP from 11 HIV-negative people who had taken two of the following nukes:
Additionally, some participants took the protease inhibitors lopinavir and ritonavir, sold as a fixed-dose combination pill called Kaletra.
Technicians found that there were no significant differences in telomere length before, during and after PEP. This suggests that short-term exposure to nukes does not have a significant effect on telomeres.
A note on children
In a separate study, Canadian researchers led by Hélène Côté, PhD, in British Columbia, have investigated the impact of nukes on the telomeres of children (both HIV positive and HIV negative) born to HIV-positive mothers and compared them to telomeres of children born to HIV-negative mothers. They found that, overall, there were no differences in telomere length. For babies exposed to nukes during their mother’s pregnancy, this confirms the findings from the Australian team that short-term exposure to nukes does not affect telomere length.
The team later assessed telomere lengths in HIV-positive children up to 19 years of age. They found that those children who had detectable viral loads tended to have shorter telomeres. This suggests that HIV itself, rather than ART, may have shortened the telomeres.
Results—Experiments with HIV-positive people
Back to the Australian study: Researchers obtained cells of the immune system from 36 HIV-positive ART users. These participants had been taking ART for at least a year and during that time their viral load was less than 50 copies/ml. Researchers also obtained cells of the immune system from 42 healthy HIV-negative people of similar age to the HIV-positive group.
The researchers found that levels of telomerase were reduced in the HIV-positive group. Furthermore, telomeres were significantly shortened in the cells of HIV-positive people.
Researchers also found that older HIV-positive people had shorter telomeres compared to HIV-negative people of the same age.
Making sense of the findings
1. Due to built-in limitations of the study’s design (it was cross-sectional in nature), the Australian researchers were not able to prove that exposure to nukes, particularly tenofovir, was the principle cause of shortened telomeres found in cells of the immune systems of HIV-positive people.
2. In some experiments, the number of samples tested was relatively small, so these results require confirmation in another study.
3. The Australian researchers focused on telomere length. However, tests are required to prove that shortened telomeres in ART users have an effect, such as a reduced capacity of the immune system’s cells to carry out their functions. Such assessments were not done by the researchers.
4. The cells of the immune system that were tested (mostly T-cells) were from what immunologists call the periphery—the blood. The vast majority of the body’s T-cells are found in lymph nodes and lymph tissues and not in the blood. Moreover, T-cells in the blood are often at the end stages of their life cycle and may not be fully functional.
A more interesting (and time-consuming, expensive and, for volunteers, somewhat painful) experiment would have been to assess telomere length using cells of the immune system taken from lymph nodes and tissues, perhaps even the bone marrow or thymus gland. Cells from such locations would be more likely to undergo replication, so telomere length would be very important for these cells.
5. The study did not take into account certain other factors in participants that could have shortened telomeres, such as the following:
- the level of immune activation
- co-infection with members of the herpes virus family, such as CMV and EBV
- tobacco smoking
- the amount of exercise they did
Also, the use of cholesterol-lowering medicines (commonly called statins) can have an anti-inflammatory effect. Many HIV-positive people take statins and this could have affected the interpretation of the study’s results.
6. Tenofovir has been available in many high-income countries for at least the past decade. It is a common part of ART. If tenofovir were prematurely aging the cells of the immune system, there should be a high proportion of tenofovir users developing cancers and serious, even life-threatening, infections. Yet there are no reports of such problems in long-term ART users arising from exposure to tenofovir. This is perhaps the most obvious and practical counterpoint to the Australian research.
Based on the results of the Australian study and its limitations, there is no data to support not prescribing tenofovir or for HIV-positive people to stop using this important medicine.
For the future
The present Australian study is interesting but future studies exploring the issue of aging, HIV and exposure to nukes and other medicines need to do at least the following:
- monitor ART users for longer periods
- assess the impact of nukes (and other drugs) on cells that are not at the end stage of their life cycle; such cells are found in lymph nodes, lymphatic tissue, bone marrow, the thymus gland and so on
- examine the effect of anti-HIV drugs on the functioning of the immune system’s cells
- determine the overall impact of nukes on the aging of cells from other organ-systems such as muscles, nerves, fat, kidneys, liver, etc.
- explore the impact of nukes on the energy-producing parts of cells (the mitochondria) in different organ-systems
—Sean R. Hosein
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